1. Field of the Invention
The present invention relates to microwave motion sensing and ranging, and more particularly to single-sideband pulse-Doppler radar including an active reflector system.
2. Description of Related Art
The Doppler effect is used to detect or measure motion in police radars, security radars and automatic door radars. Microwave vibration sensing, while similar to Doppler sensing, relies on target-induced phase modulation of a radar return, and has generally been limited to specialized industrial/scientific use. One well-publicized exception was the microwave illumination of the U.S. embassy in Moscow during the Cold War, where voices may have caused a subtle vibration in reflecting objects that could be intercepted by a vibration-sensing microwave receiver outside the embassy.
Vibration sensing radars show great potential in medical diagnostics/monitoring and speech recognition, such as cardiac monitoring (e.g., xe2x80x9cBody Monitoring and Imaging Apparatus and Method,xe2x80x9d U.S. Pat. No. 5,573,012 to McEwan, 1996), respiration monitoring (e.g., xe2x80x9cRespiration Monitor,xe2x80x9d U.S. Pat. No. 3,993,995 to Kaplan et al, 1976), and vocal cord sensing (e.g., xe2x80x9cSpeech Coding, Reconstruction and Recognition Using Acoustics and Electromagnetic Waves,xe2x80x9d U.S. Pat. No. 5,729,694 to Holzrichter et al, 1998).
One problem in sensing vibrations with radar is the occurrence of periodic nulls in the sensing field located every xcex/2 of the RF wavelength. Thus, it is possible to fail to detect a vibrating object depending on its range from the radar. This problem is present in both single phase and quadrature phase radars. It can only be eliminated with SSB radar.
The direction of motion can be sensed by Doppler radars outfitted with quadrature detectors. A further adjunct to direction sensing is displacement sensing, where a direction-signed motion signature is integrated over a number of Doppler cycles such that a definite target displacement in one direction must occur before a detection threshold is reached (e.g., xe2x80x9cIntrusion Detection System,xe2x80x9d U.S. Pat. No. 3,942,178 to Hackett, 1976).
Displacement sensing overcomes the hair-trigger nature of a motion sensor by requiring a substantial target movement before triggering. Displacement sensing also reduces another false alarm nuisance: trigging on RF interference. It would be very difficult for RF interference to appear like it is moving in one direction only, i.e., appear as a single sideband to a microwave carrier that is consistently within perhaps 100 Hz of a 10.5 GHz carrier-generally, neither the radar nor the interference would have such coherence or stability. Unfortunately, prior art displacement sensors do not implement interference rejection in a robust fashion, nor do they fully demodulate the Doppler sidebands into separate analog channels.
Prior art Doppler radars are lacking in key features needed to enable innumerable applications, e.g., they lack dual channel analog single sideband demodulation combined with short-pulse range-gating while implemented in a low cost architecture. Also, the prior art does not teach the use of an active reflector to measure range or material thickness from the phase relation between the range-gated Doppler sidebands.
The present invention is a system comprised of a homodyne pulse Doppler radar employing SSB demodulation which can be used either separately or in combination with an active reflector, which may also have SSB capability.
The radar transmits RF pulses, or sinewave packets or bursts, at a pulse repetition frequency PRF. As used herein, the entire burst or packet is referred to as an RF pulse, with each pulse containing one or more cycles of a sinusoidal waveform at a carrier frequency. The transmitted pulses and receive echoes are applied in quadrature to a pair of peak-hold detectors that have a large hold time spanning multiple pulse repetitions. The RF pulse width may be 30 ns wide at a 915 MHz carrier frequency with a corresponding RF bandwidth of about 30 MHz. By integrating pulse repetitions over a long time, e.g., 1-second, Doppler sidebands only 0.1 Hz removed from the RF pulse carrier can be detected.
The maximum range, or range gate, is defined by the RF pulse width, since the RF peak detectors operate only while the transmitted pulse is present, such that echoes that arrive after the transmitter RF pulse ends do not get peak detected.
The RF peak-hold detector outputs are phase shifted and algebraically summed to provide upper sideband (USB) and lower sideband (LSB) outputs. USB outputs occur with objects moving toward the radar and LSB outputs occur with objects moving away from the radar.
One feature of the invention is that the USB and LSB outputs (or channels) can be rectified and filtered to provide a directional displacement indication. These indications can be combined in a weighted fashion such that a small amount of displacement in the wrong direction, for a given LSB or USB channel, will effectively reset that channelxe2x80x94an especially useful feature for eliminating false alarms from loitering or RF interference.
When sensing vibrations, either the USB or LSB channel provides an output free of nulls that occur every xcex/2 with non-quadrature radar or in the I or Q channel of a quadrature radar, thus removing a barrier to practicality in many applications.
Stereo operation as provided by the LSB and USB channels is non-obvious and quite unexpected in many applications such as picking up guitar string vibrations, where each stereo output has the same general frequency of the vibrating string but a different tonal quality, providing a rich, airy sound on stereo headphones. Other vibration sensing applications include (1) sensing machine vibration or motion, including blade rotation with rotational direction indication, (2) direction sensitive cardiac motion sensing, (3) respiratory motion sensing, and (4) a vocal cord microphone.
When detecting a vibrating object, the phase between signals at the LSB and USB outputs varies 360 degrees with every xcex/4 of distance from the vibrating object. For an RF center frequency of 915 MHz, 360 degrees represents a distance of 8.2 cm. Since the phase signals are very clean, it is an easy matter to measure sub-millimeter variations in distance, or equivalently dielectric constant or material thickness of objects inserted between the radar antenna and the source of vibration. One application of this effect is water or oil level measurement in a tank, by measuring the transit time through the height of the liquid. The advantage to this technique is that low frequency RF can be used without the need for a large focusing antenna, since the measurement is confined to reflections from the vibrating object, which is usually the only object in the field of view that is vibrating.
Changes in radar cross section (xcex94-RCS) can be detected with the present invention, such as might occur when two metallic objects scrape against each other. If two smooth tweezers are slid across each other near the radar antenna, a large noise-like response is produced by the radar indicating the degree of surface smoothness. The xcex94-RCS technique can also be used in combination with mechanical or acoustic pressure to locate gold nuggets in quartz, such as might be found in a gold mine.
Electronic objects that contain switching circuits or variable conduction devices, such as audio transistors, also provide a xcex94RCS signature that is readily detected by the present invention. Applications include non-contact electronic circuit readout, toys that xe2x80x9ctalkxe2x80x9d upon illumination by a radar, and RF identification (RFID) tags.
The xcex94-RCS effect can be deliberately enhanced by designing a reflector that undergoes a large xcex94-RCS under electronic control. Accordingly, another aspect of the present invention is a modulated dipole or other reflector/antenna, wherein the modulation is voice, music, continuous tones or data. The modulation can be provided by a very low power CMOS circuit, such as the type used in wrist watches, and the modulating device may be a FET, which requires essentially no current to drive its gate. The micropower active reflector thus formed emits no RF energy and is thus not subject to emission regulations, and yet it can transmit information when illuminated by the radar of the present invention.
The active reflector can be quadrature modulated to provide a SSB reflection that appears in either the LSB or USB channel of the illuminating radar. Thus, multi-channel operation is possible, beyond the usual code or frequency division techniques.
An RF beam interruption system can be configured with a radar and active reflector by forming a beam path through which passing objects will create an interruption. Advantages include (1) the ability to operate through panels and environmental overcoatings such as snow, and (2) the ability to freely locate micropower active reflectors as stick-on appliques. When solar powered, the active reflectors can be permanently attached to trees, lampposts, swimming pool gates, garage door frames, etc. for simplified security, safety, or home automation control.
A system of active reflectors can be combined with the radar to form a radiolocation system wherein the range to each reflector is measured by the radar and used to calculate the location of the radar and/or one or more reflectors.
The entire radar system of the present invention can be implemented with low cost off-the-shelf components. The applications cited herein are but a few of the many novel uses for this invention.